In recent years, with growing demand for highly-functional electronics, printed wiring boards capable of high-density mounting are required to miniaturize and increase density, and there is a tendency that width of conducting circuits or width between conducting circuits formed on the printed wiring boards further narrow. For example, in inner layer circuit boards used for multilayer printed wiring boards, the width of conducting circuits and width between conducting circuits (L/S) have a tendency to narrow from conventional 100 μm/100 μm to 50 μm/50 μm, and further to 20 μm/20 μm. Therefore, practical application of outer layer circuits of the multilayer printed wiring boards having the L/S of 15 μm/15 μm have been particularly studied.
When the L/S narrows, insulating layers used for the multilayer printed wiring boards is required to have excellent reliability than ever before. Specifically, the insulating layer comprising a resin composition such as an epoxy resin composition is required to have lower thermal expansion coefficient, and it is further required to have higher heat resistance, flame resistance and insulation reliability.
However, resin compositions used for the insulating layers of the conventional printed wiring boards are not capable of exhibiting a low thermal expansion coefficient and higher heat resistance, flame resistance and insulation reliability than ever before, even if fine wiring is formed on the printed wiring board (for example, Patent Literatures 1 and 2).